A Passive Optical Network, PON, is a point-to-multipoint network architecture employing fibre cables from a central office to premises. One such PON is a Wavelength Division Multiplexed PON, WDM-PON.
It employs an Arrayed Waveguide Grating, AWG, to enable a single optical fibre to serve multiple premises. The AWG is used as an optical (de)multiplexer in the WDM-PON. The AWG is capable of multiplexing a large number of wavelengths into a single optical fibre, thereby increasing the transmission capacity of the network. A WDM-PON comprises an Optical Line Terminal, OLT, at the central office of the service provider. It comprises a number of Optical Network Terminals, ONTs, near end users. A WDM-PON configuration reduces the amount of fibre and central office equipment required compared with point-to-point architectures. A passive optical network is a form of fibre-optic access network.
A PON, such as a WDM-PON should be supervised and monitored in order to detect any possible occurring failure in the WDM-PON. No supervision results in that a failure in the WDM-PON is recorded only based on service provision drop-out, resulting in loss of revenue for the operator. In order to make PONS and also WDM-PONS attractive to the operators, by reducing their Operational Expenditures, it is important to develop cost-efficient, fully reliable and accurate monitoring solutions which support fault detection, identification and localization of a possible fault in different fibre-access topologies.
This is of particular importance in WDM-PONS because a single WDM-PON will most likely support a lot of bandwidth-demanding end users. One advantage of a WDM-PON is that can provide symmetric bandwidth per user or per wavelength of 1-10 Gbit/s at minimum. Reliability and short downtimes are crucial for the end users to experience good quality and for the operators not to suffer loss of revenues.
Preventive monitoring of a PON may provide information to an operator on foreseeable problems, e.g. a decrease of received power at an ONT, degradation or malfunction of a connector or splice, aging or bending of a fibre in the PON and so on. Thus preventive monitoring can help mitigate potential signal losses. Furthermore, in case of sudden malfunction, a monitoring system should be able to timely detect and localize faults. Remote and automatic monitoring contributes to Operational Expenditure savings as no technical crew has to be dispatched to the field until the moment when the failure is defined and localized.
Some requirements can be defined for a monitoring system. For example, monitoring should not influence regular data communication. In other words, the monitoring should be non-invasive. This is achievable by utilizing a dedicated optical bandwidth. Furthermore, the monitoring should be sensitive to relatively small power fluctuations detectable in on-demand mode or in periodic mode. On top of that, it should not require any high initial investment.
In order to supervise and monitor the performance of a PON, Optical Time-Domain Reflectometry, OTDR, can be used. An OTDR device injects a series of optical pulses into the fibre. The series of optical pulses, also called OTDR signals travel down the network towards the ONTs. Parts of the OTDR signals are reflected back towards the OTDR device. The back reflected, or backscattered, OTDR signal may be used for estimating the fibre's length and overall attenuation, including splitter losses. The backscattered OTDR signal may also be used to locate faults, such as breaks, and to measure optical return loss.
Most of the methods or techniques known today satisfy only some of the above requirements. Most the methods or techniques known today increase Capital Expenditures significantly since they require either tuneable or multi-wavelength OTDR. Tuneable or multi-wavelength OTDR requires larger bandwidth to accommodate multi OTDR channels. Some of the methods or techniques known today require advanced OLT transmitter upgrades which provide service-disruptive monitoring. Some other methods require significant optical distribution network upgrades due to the duplication of parts of the light paths. Furthermore, most the methods or techniques known today can only detect a fibre fault which introduces significant loss, far above the expected threshold of 1 dB. Still further, the majority of methods can only detect a fault and determine the faulty branch, i.e. the fibre from a remote node to an ONT, and they are not able to determine the exact location of the fault, e.g. the distance from the remote node.